1. Field of the Invention
This invention relates to a sheet feeding roller, mounted to a sheet feeding section for feeding sheets for image forming apparatuses such as printers, photocopiers, and the like, for feeding sheets by rotating in pressed contact with stacked sheets and to a sheet feeding apparatus and an image forming apparatus having this roller.
2. Description of Related Art
Referring to FIG. 9, a sheet feeding roller for feeding such as original documents and recording sheets in a conventional image forming apparatus such as printer or photocopier is described. As shown in FIG. 9, in a sheet feeding roller 100, a housing 102 for supporting a rubber 101 serving as an elastic body is secured to a rotary shaft 103 to which drive is transmitted, and sheets are fed sheet by sheet according to rotation of the rotary shaft 103. The sheet feeding roller 100 is in a half moon shape having a cutout 104 at a part thereof, and at the home position while in waiting state, the cutout 104 faces to the sheets and does not contact with the sheets. An arc portion ensures an adequate length allowing one sheet to be picked up and delivered to the subsequent roller, not shown, disposed on a downstream side. Grooves and the like are formed on a surface of the sheet feeding roller 100, thereby devising the roller as to improve the friction force from the shape.
Sheets are stacked on a special feeding cassette 105. An intermediate plate 106 for placing the sheet bundle thereon and a spring 107 for urging the intermediate plate 106 toward the sheet feeding roller 100 are formed inside the feeding cassette 105. A separation pawl 108 is formed on a downstream side of the sheet feeding roller on the sheet feeding cassette 105, 100 in the sheet feeding direction, corresponding to the corner of the front end of the sheet placed on the intermediate plate 106, thereby separating sheets one by one by forming a loop to the sheets to some extent.
If the sheet feeding operation begins, the rotary shaft 103 starts rotating, thereby rotating the sheet feeding roller 100, and then, the roller 100 comes in pressed contact with the sheets in pushing down the sheet bundle at a portion where the sheet feeding roller 100 transits from the cutout 104 to the arc portion. The rubber 101 serving as an elastic body contacts to the topmost sheet of the sheets in applying a proper contacting pressure and feeds the sheet. At that time, a loop is gradually formed upon which the corner of the front end of the topmost sheet is made to urge the separation pawl 108, and finally the corner of the front end of the sheet disengages from the separation pawl 108 thereby separating solely the topmost sheet.
With such a sheet separation method in use of the separation pawl method, the contact pressure N applied between the sheet feeding roller 100 and the sheets is very important. If the contact pressure N is too weak, the sheets cannot overcome the opposition force from the separation pawl 108 when the sheets are urge against the separation pawl 108, so that slipping is made between the sheet feeding roller 100 and the sheets and that the sheets cannot be fed smoothly. If the contact pressure N is too strong, large frictional force may occur between the topmost sheet and the sheet just below the topmost sheet or sheets further below the topmost sheet, so that double or triple sheets may be fed instead of feeding of a single sheet.
To avoid such a problem, normally the spring 107 for lifting up the intermediate plate 106 is made with a proper load setting and a spring coefficient setting. More specifically, the weight of sheets changes according to the stacked number, and according to the stacked number, the contraction amount of the spring 107 also changes for lifting up the intermediate plate 106. Using this relation of both, the spring coefficient is set so as to make approximately constant the load exerted to the sheet feeding roller 100 and the sheets even where the stacked number of the sheets is changed.
With the structure according to the conventional art, if the kind of the sheets set to the sheet feeding cassette 105 is unchanged, the load exerted between the sheet feeding roller 100 and the sheet can be an optimum value as the theory described above, but in fact, the size and density of the stacked sheets may be diversified to many kinds. There is no spring to give constantly the optimum load to these various sheets, and the reality is that a spring having a proper value is selected in evaluating the whole balance with respect to the various sheets.
According to recent colorization of image forming apparatuses such as printers, it is a trend that users favor sheets having thicker thickness. On the other hand, from the reason to avoid environmental destructions, sheets having thinner thickness are also used more frequently. Thus, demands on the sheets allowed to be fed from the sheet feeding cassette 105 are made various, and it is difficult to correspond all the variety by solely optimizing the spring 107. That is, if the design targets at the thick paper, the spring load described above is set strongly, and therefore, doubly feeding failures (in which sheets are fed in a stacked manner) may occur in the case of thin paper although feeding failures (i.e., a phenomenon in which sheet feeding roller 100 slips on the sheet and is unable to feed out the sheet) may not occur. To the contrary, if the design targets at the thin paper, the spring load is set weakly, thereby raising a problem such that feeding failures may occur in the case of the thick paper even while doubly feeding failure may not occur.
It is an object to provide a sheet feeding roller automatically adjusting contact pressure exerted between the sheet feeding roller and sheets during feeding operation and being capable of feeding sheets properly and surely, and to provide a sheet feeding apparatus and an image forming apparatus having the above sheet feeding roller.